If someone has 25000 lifetime posts from 2001 and 1500 the ratio is 1.6

But since the function started after they already had 15,000 posts counting the 10K posts after 2010 *eligible* for against the 1500 works out to 6.66

Their ratio from day one til active, and yours would be 0. So you've got 15K posts of 0 that weren't eligible for included with the 10 K posts of making the difference 1.6 versus 6.66 That's a pretty big difference eh?